This invention relates to an electric double layer capacitor constructed of a plurality of concentric rings of capacitor pairs for use as an electrochemical device for energy storage or deionization of liquids.
Deionizing liquid streams, and in particular aqueous streams, has very significant importance in the world today. An increasing portion of the fresh water supply for the world is coming from desalination plants that are current operated with reverse osmosis systems. These systems require a tremendous amount of energy, high maintenance due to the extreme operating pressures, and chemicals to remove fouling from the reverse osmosis cylinders. Other large opportunities for deionizing water are industrial softening for water towers, processing of water by-products from the oil and gas industry, and residential water softening. These opportunities are current being addressed through ion exchange with resins or sodium chloride and standard waste water treatment precipitation.
Capacitive deionization devices have been developed over the last 20 years as a possible replacement for the existing methods. Capacitive deionization in general has the ability to remove ions with lower energy and minimal fouling. Unfortunately, the devices produced and patented suffer from a number of limitations listed below.
Capacitive deionization works as follows. An aqueous stream containing undesirable ions is fed into a device containing one or more pairs of electric double layer capacitors. A power supply is attached to the pairs and the capacitors are charged. Since there is a dielectric material or layer in between the layers, they hold their charge just like a standard capacitor.
When charged “positively”, the cations and anions are removed from solution and adsorbed onto a capacitor which is typically made of carbon. The carbon capacitors, or capacitors, eventually fill with ions. When this occurs, the polarity of the double layer capacitor is switched and the ions are ejected from the surface of the carbon into the stream and carried out of the device.
Unfortunately, the timing and space constraints of existing devices do not allow for a clean separation between the cleaned stream and the following concentrated stream. Because these two streams partially mix together, the purification ability of the device is limited.
Current capacitive deionization devices have significant limitations for performance due to the design constraints employed. In all cases, the devices are difficult to assemble, suffer from the effect of large dead volume spaces within the devices, and other performance limiting issues which will be described in detail below.
In U.S. Pat. No. 5,192,432 to Andelman, 1993 Mar. 9, had a spirally wound electric double layer capacitor with no charge barrier and a large internally exit tube that allowed for mixing of the cleaned and dirty streams. The lack of the charge barrier allows discharged ions to re-adsorb onto the opposing capacitor and the large exit tube volume allows for mixing of the cleaned and dirty process streams. Also, the spirally wound design causes a large linear path for the water, which increases the residence time in the device and increases the difficulty of separating the clean from dirty process streams.
Another type of capacitive deionization device is the use of a flat plate design in which electric double layer capacitor pairs are stacked one on top of the other, creating a sandwich of one or more pairs. The flat plates can be circular as in U.S. Pat. No. 5,200,068 to Andelman, 1993 Apr. 6 and U.S. Pat. No. 5,360,540 to Andelman, 1994 Nov. 1.
In either case, both designs suffer from a large dead volume of space within the device where streams can be mixed during the change between purification and purging cycles. This limitation is discussed by Andelman in the attached publication presented to the International Workshop on Marine Pollution on Dec. 1-3, 2003 on page 11.
Because of the rigid casings of both the spiral and flat plate designs, it is difficult to adjust the performance parameters of the device. For example, it would be very difficult to add or subtract capacitor pairs from the flat plate design without changing the dimensions of the casing.
The common flat plate design also suffers from the inability to control the amperage draw of the capacitor thereby reducing the time window in which to separate clean from dirty streams.
These design issues prevent the current capacitive deionization devices from being operated in series allowing the water to pass from one to the next until an extremely clean stream emerges from the last cell. Storage tanks must be placed in between stages.
The existing capacitive deionization designs also suffer from precipitation of low solubility ions and must be periodically flushed with chemicals to remove the fouling. This is especially true with the flat plate design.
The existing designs also utilize a porous current collector which is difficult to assemble and imparts additional electrical resistance to the system.
The spirally wound design is difficult to assemble and has a large operating pressure drop through the device due to the tortuous path the liquid must follow. The capacitor pair must be continuously wrapped around the large perforated core without tears or gaps that the water could pass through unprocessed.
The flat plate design layers must be stacked individually until the desired height is reached. The alignment is critical at each end and in the center where the processed liquid exits. The compression under which the stack is compressed is difficult to control.
All cited capacitive deionization devices specify to operate at less than 1.5V. This reduced operating voltage lowers the potential capacity of the device by upwards of 30%.
Because of the design limitations, it is difficult to control the output concentration of the device which is the primary purpose of any deionizing system.
The amount of ions that can be adsorbed onto the surface of the carbon is exactly equal to the electrical capacitance of the capacitor in use. The current design of capacitors has a limited capacity due to the design and therefore limits the amount of ions that can be adsorbed in a given cycle and speed in which the ions are removed from solution.
Existing capacitive deionization devices have a circumference to length ratio of approximately 2.5. The radial design generally has a ratio closer to 0.25:1. This increased residence time allows for difficult ions to be removed by removing the easier ions in the first part of the device, leaving the harder to remove ions available only in the electromagnetic field.
Existing designs rely on a series of connections between poor conductivity materials and the power supply.
These and other advantages of one or more aspects will become apparent from a consideration of the ensuing description and accompanying drawings.